Gasoline is one of most commonly used fuels for motor vehicles. Gasoline has a relative low vapor pressure under normal condition, and vaporizes to a significant extent in the fuel tank depending on the temperature and pressure of the gasoline within the fuel tank. The fuel vapor is required to be vented in order to control the internal pressure of the fuel tank within a prescribed level, and the vented gasoline vapor is recovered by a canister filled with charcoal or other absorbent substance in order to avoid polluting the atmosphere. The fuel vapor captured in the canister is conducted to the intake system of the engine to be burned in the engine.
The canister is connected to the fuel tank via a pipe, and the fuel tank end of the pipe is connected to an upper part of the fuel tank to avoid liquid fuel from being forwarded to the canister. However, the vehicle is often subjected to violent motions due to irregular road surfaces, and the accelerations/decelerations and cornering movements of the vehicle, and may be parked on a slanted road surface. Therefore, the fuel tank end of the pipe leading to the canister is provided with a cutoff valve which is provided with a float valve configured to close the communication to the canister when the liquid fuel level rises beyond a certain level.
The fuel tank is additionally provided with an arrangement for preventing the overfilling the fuel tank that includes a float valve. This float valve closes when the liquid fuel level reaches a certain level, and causes the internal pressure of the fuel tank to rise. This causes the liquid fuel to rise in the filler pipe, and this in turn causes the fill limit sensor of the fuel pump nozzle to be activated. This arrangement controls excessive filling of fuel in the fuel tank, and ensures an adequate gas phase space within the gas tank. It has been proposed to combine the float valve for controlling the communication with the canister and the float valve for preventing the overfilling of the fuel tank into a single integrated valve. See Japanese patent No 3931291 (patent document 1) and Japanese patent 3948194 (patent document 2), for instance.
In the float valve disclosed in patent document 1, a small opening is formed in a case (housing) receiving a float therein to communicate the interior of the fuel tank with a pipe leading to a canister. This small opening is provided at a height higher than the liquid fuel level when the fuel tank is filled up so that even when the lower opening of the housing is closed by the liquid fuel, the fuel vapor is allowed to be vented to the canister.
In addition to the small opening, the housing of the float valve is provided with a skirt extending vertically in a lower part thereof. Thereby, when the liquid fuel level exceeds the level of the lower opening of the skirt, an increase in the internal pressure of the fuel tank causes the fuel liquid level within the housing (or skirt) to rise higher than the liquid fuel level of the remaining part of the fuel tank, and this in turn causes the float to rise. Once the float has risen high enough to close the port communicating with the canister, the internal pressure of the fuel tank rises sharply, and this activates the fill limit switch of the fuel pump nozzle. Thereafter, the fuel vapor is gradually introduced into the housing via the small opening, and this causes the pressure in the housing to drop. This in turn causes the float to drop and the communication with the canister to be established again. If this occurs too soon, the liquid fuel level in the filler pipe may drop too quickly and the fill limit sensor of the fuel nozzle to be released depending on the way the fuel is filled so that overfilling of the fuel tank could occur.
The problem of overfilling can be avoided by reducing the opening area of the small opening in the housing as this would delay the dropping of the liquid fuel level in the housing (hence the opening of the port leading to the canister). However, to ensure fuel vapor to be forwarded to the canister at an adequate flow rate, the opening area of the small opening is required to be reasonably large. If the opening area of the small opening is too small, the pressure inside the housing can be readily reduced, and the resulting rise in the liquid fuel level in the housing causes the float valve to close with the result that the fuel vapor is prevented from being vented to the canister at an adequate flow rate.
Also, there is an increasing demand to reduce the height of the fuel tank for the purpose of efficiently utilizing the space available in a vehicle, and this in turn requires the height of the float valve to be minimized. This means a reduced height for the skirt, and this increases the sensitivity of the float valve to the change in the liquid fuel level in the housing. In other words, it becomes more difficult to ensure proper venting of fuel vapor and to prevent overfilling of the fuel tank at the same time.